1. Field of the Invention
The present invention relates to a gas barrier film that is suitable for a packaging material, which requires air-tightness and oxygen barrier properties, to be used for foods, medical drugs, electronic parts and the like.
2. Description of the Related Art
Heretofore, a polypropylene film excellent in water vapor barrier properties has been used as a transparent gas barrier film for packaging, and especially when high oxygen barrier properties are required, the polypropylene film is subjected to various kinds of surface processing. Examples of the surface processing include the following methods: (1) a resin having relatively high gas barrier properties, such as polyvinylidene chloride, polyvinyl alcohol or an ethylene/vinyl alcohol copolymer, is coated on a surface of the polypropylene film, or alternatively, a film of the resin having relatively high gas barrier properties is laminated on the polyethylene film to form a multilayer structure including the resin film and the polypropylene film; (2) an aluminum foil is laminated on the surface of the polypropylene film, or alternatively, aluminum is vacuum-deposited to form a metallic thin film on the polypropylene film; and (3) an inorganic compound (e.g., metallic oxides such as aluminum oxide, and silicon oxide) is vapor-deposited on the surface of the polypropylene film to form a thin film of the inorganic compound.
A polypropylene film having undergone the surface processing (1) can be versatilely used due to transparency, processing suitability and cost thereof. However, the gas barrier film obtained by coating polyvinylidene chloride on the polypropylene film in the surface processing (1) has a problem in that a hydrogen chloride gas is generated when the film is disposed of by burning causing a damage to an incinerator and, under some burning conditions, possibly leading to environmental pollution. Although the film obtained by the surface processing (1) by coating polyvinyl alcohol or an ethylene-vinyl alcohol copolymer does not pose a problem associated with burning, the film is insufficient in gas barrier properties with respect to oxygen and water vapor under conditions of high temperature and high humidity owing to the hygroscopic nature thereof, and accordingly, its applicability is limited.
A polypropylene film having undergone the surface processing (2) is excellent in appearance and gas barrier properties with respect to water vapor and oxygen, although the film is lacking with respect to visibility of contents. However, this gas barrier film does not transmit microwaves and hence cannot be used in a microwave oven. Furthermore, this film has problems in that the aluminum foil accounts for a large proportion of production costs of the packaging material, and aluminum remains as a mass after the film is burnt. Furthermore, when the aluminum foil is used, due to a thickness of several tens of micrometers that is required for excellent gas barrier properties, a heaviness of the film makes it unsuitable for use as a packaging material.
A polypropylene film having undergone the surface processing (3) has been widely used in recent years owing to its transparency and light weight. Such a film that is obtained by simply vapor-depositing an inorganic compound (e.g., metallic oxides such as aluminum oxide, and silicon oxide) on the surface of a polypropylene film preferably has an increased thickness to achieve sufficient oxygen barrier properties. However, the film having increased thickness has problems of poor flexibility, a tendency to develop color leading to opaqueness, and higher film forming costs. Furthermore, there may arise many problematic cases where adhesion between the polypropylene film and the inorganic compound thin film is insufficient causing peeling of the inorganic compound thin film or generation of cracks, thereby resulting in deteriorated gas barrier properties.